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Effects of Cooling Rate on the Solidification and Microstructure Of materials Article Effects of Cooling Rate on the Solidification and Microstructure of Nickel-Based Superalloy GTD222 1 1 1 1 1 2, , Bo Gao , Yanfei Sui , Hongwei Wang , Chunming Zou , Zunjie Wei , Rui Wang * y 2,3, , and Yanle Sun * y 1 National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China; [email protected] (B.G.); [email protected] (Y.S.); [email protected] (H.W.); [email protected] (C.Z.); [email protected] (Z.W.) 2 Shanghai Key Laboratory of Advanced High-Temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 3 School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China * Correspondence: [email protected] (R.W.); [email protected] (Y.S.) These authors contributed equally to this work. y Received: 6 May 2019; Accepted: 12 June 2019; Published: 14 June 2019 Abstract: In this work, the microstructure and solidification behavior of nickel-based superalloy GTD222 at different cooling rates are studied. The solidification of the superalloy GTD222 proceeds as follows: L L + γ,L L + γ + MC, L L + (γ/γ )-Eutectic and L η phase. Due to alloying element ! ! ! 0 ! redistribution, the temperature of the solidus GTD222 superalloy, 1310 ◦C, is slightly lower than the temperature of the liquidus, which is 1360 ◦C. It was found that the dendrite arm spacing of the alloy decreased with the increase of the cooling rate from 200 µm at 2.5 K/min to 100 µm at 20 K/min. Keywords: GTD222; nickel-based superalloy; solidification behavior; cooling rate 1. Introduction Casting superalloys are widely applied in industrial areas such as gas turbines, aerospace and chemical process industries owing to their excellent mechanical properties and thermal corrosion resistance [1,2]. In order to develop more efficient advanced solidification technology, a data base of superalloys on thermophysical properties is increasingly needed [3]. From a theoretical and industrial perspective, knowledge of superalloy solidification behavior is crucial for the controlling of the superalloy casting process [4]. In the casting of nickel-based superalloys, the mechanical properties of alloys are due to microstructure characteristics, such as a combined matrix γ phase and γ0 precipitation phase [5], dendritic width [6] and grain size [7]. Hence, the optimum mechanical properties can be obtained only by applying suitable heat treatment [3]. For heat treatment after casting, the solidus temperature (the temperature at which incipient alloys begin to melt) and the formation temperature of precipitation (the onset precipitating of the γ0 phase, η phase and carbide phase), which determine the heat treatment window of materials, are very important. To obtain the ideal microstructure, heat treatment of the nickel-based superalloys must be carried out in certain temperature range. Therefore, for any nickel-based superalloy, solvus temperature and solidus temperature should be accurately measured, in order to optimize the mechanical properties of the superalloy. What is more, the incipient dissolving temperature of precipitation is another important parameter, helping to maximize the volume of precipitates without producing an interdendritic region. In recent years, the demand of hot-end complex parts with different wall thicknesses has increased. The casting system of the hot-end parts is very intricate, and that leads to difficulty in controlling Materials 2019, 12, 1920; doi:10.3390/ma12121920 www.mdpi.com/journal/materials Materials 2019, 12, 1920 2 of 9 the microstructure. Numerous studies have shown a direct relationship between the cooling rate and material microstructure, such as the significant effect on the dendritic width by the cooling rate of solidification process. Chen et al. [8] studied the compositional changes of the micro-scale precipitates of an advanced Ni-base superalloy at different cooling rates. It was found that the chemical composition of the precipitates of different sizes is very different. This study has important implications for understanding the microstructure and precipitation behavior of Ni-based superalloys. Zheng et al. have testified that the cooling rate significantly influences the morphology of dendrites [9]. The dendrite arm spacing of both primary and secondary dendrite declined as the cooling rate increased [10–12]. GTD222, a nickel-based precipitation hardened isometric crystal superalloy, is considered to be one of the most suitable superalloys that can be processed into the guide vane of a steam turbine, servicing at 1000 ◦C[13]. Most work focused on the optimization of the composition of the GTD222 superalloy. However, less attention has focused on the solidification behavior of the GTD222 superalloy [14]. In this study, an empirical research was carried out to understand the effect of cooling rate on the solidification behavior and microstructural evolution of the GTD222 superalloy, and the liquidus temperature and solidus temperature of the GTD222 superalloy were also measured. 2. Experimental Procedures The chemical composition of the GTD222 superalloy used in the present work is given in Table1. Commercial pure metals (> 99.95 wt.%) were used for the preparation of the alloy prior to melting. To ensure compositional homogeneity, the alloy melt was fully stirred by an electromagnetic stirring system equipped in the arc furnace (QSH-ZP, Quanshuo, Shanghai, China), and each button alloy was flipped and melted at least four times. The materials were firstly prepared in a vacuum induction melting furnace, and then casted into ingots (100 mm 100 mm 150 mm). All specimens used in this × × work were cut from the ingot using a spark cutting machine (Dk77, Zhonggu, Suzhou, China). Table 1. Chemistry (wt.%) of the GTD222 superalloy. C Cr Co W Al Ti Nb B Ta Ni 0.08–0.12 22.2–22.8 18.5–19.5 1.8–2.2 1.0–1.4 2.1–2.5 0.7–0.9 0.002–0.007 0.9–1.1 Bal. The solidification procedure of the GTD222 superalloy was revealed by differential scanning calorimetry (DSC, DSC 404 F3 Pegasus, NETCH, Selb, Germany). All DSC testes were carried out by alumina crucibles in the argon protection environment with sample sizes of 2.5 mm in diameter and 2 mm in height. The cooling rates of samples were 2.5 K/min, 5 K/min, 10 K/min and 20 K/min, respectively. Microstructural evolution and phase constitutions of GTD222 samples were carried out on an optical microscope (OM, Axio Imager A1m, ZEISS, Jena, Germany), X-raydiffraction (XRD, XRD-6000 diffraction instrument, Shimadzu, Kyoto, Japan), X-ray energy-dispersive spectroscopy (EDS, JSM-7600F, Tyoto, Japan) and scanning electronmicroscope (SEM, Sirion 200, FEI, Hillsboro, OR, USA). The samples for OM and SEM were ground to 2000 grit, and then polished by the diamond polishing paste (1 µm). The etchant for samples was 45 mL CuSO4, 100 mL H2O and 50 HCl. Phase constitutions of the alloy were determined by X-ray diffraction (XRD) technique, using Cu Kα (λ = 0.1540562 nm) radiation, operating at 40 kV and 30 mA between 20◦ and 80◦ (2θ) at a step of 0.02◦ and a counting time of 0.6 s per step. 3. Results and Discussion 3.1. Microstructure of As-Cast Alloy Figures1 and2 show the microstructure of an as-cast GTD222 superalloy with di fferent cooling rates. The microstructure of the GTD222 superalloy is a typical dendritic structure in the as-casting samples, and no equiaxed grains were found in Figure1. The dendritic structures in Figure1a are coarser than dendritic structures in Figure1b–d. It can be seen that the dendritic structures are Materials 2019, 12, x FOR PEER REVIEW 3 of 9 Figures 1 and 2 show the microstructure of an as-cast GTD222 superalloy with different cooling rates. The microstructure of the GTD222 superalloy is a typical dendritic structure in the as-casting Materialssamples2019, and, 12 ,no 1920 equiaxed grains were found in Figure 1. The dendritic structures in Figure 1a3 are of 9 coarser than dendritic structures in Figure 1b–d. It can be seen that the dendritic structures are coarsening, along with the decrease in the cooling rate. Additionally, the secondary dendritic arm coarsening,spacing has alongthe same with tendency. the decrease According in the cooling to V. Kavoosi, rate. Additionally, the secondary the secondarydendritic arm dendritic spacing arm is spacingrelated hasto the the samelocal tendency.cooling rate According [15]. The to V. size Kavoosi, of the the secondary secondary dendriticdendrites arm directly spacing affects is related the tocomposition the local cooling segregation, rate [ 15the]. sec Theond size phase of theand secondary the distribution dendrites of micropores. directly aff Asects shown the composition in Figure 2, segregation,the size of the the carbide second precipitation phase and the is distribution decreasing ofas micropores.the cooling Asrate shown increas ines Figure from2 2.5, the K/min size of to the 20 carbideK/min, since precipitation a low cooling is decreasing rate would as the provide cooling more rate increases time for fromthe diffusion 2.5 K/min of to the 20 atom K/min,s and since coarsen a low coolingthe second rate phase. would Furthermore, provide more the time secondary for the didendriteffusion arm of the spacing atoms of and the coarsen alloy is theclosely second related phase. to Furthermore,tensile strength the and secondary elongation. dendrite Generally, arm spacing the smaller of the the alloy distance is closely between related secondary to tensile dendritic strength andarms, elongation. the better the Generally, mechanical the smallerproperties the p distanceroduced. between The high secondary cooling rate dendritic would arms,refine thethe bettergrain theand mechanical dendrite arm properties spacing of produced. materials, The producing high cooling a greater rate wouldfraction refine of boundaries the grain and dendriteconsequently arm spacingimproving of materials,the mechanical producing performance a greater of fraction the materials of boundaries at ambient and consequentlytemperature.
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